In the art, translucent screens are generally used in rear projection televisions. An example of the constitution of such translucent screen is shown in the schematic cross-sectional view given in FIG. 3. In FIG. 3, 1 is a Fresnel lens sheet and 2 is a lenticular lens sheet. Generally, a translucent screen is constituted of these Fresnel lens sheet 1 and lenticular lens sheet 2 in close contact with each other. The incidence side, exit side, incidence surface and exit surface, so referred to hereinafter, respectively indicate the incidence side, exit side, incidence surface and exit surface of the translucent screen to be irradiated with light for image display thereon as shown in FIG. 4.
An example of the constitution of a lenticular lens sheet 2 is shown in the cross-sectional view given in FIG. 4. As shown in FIG. 4, on the light incidence side of the lenticular lens sheet 2, there are generally arranged plural incidence side lenses 21 having a roughly semicylindrical cross section. These plural incidence side lenses 21 are formed so that the lens troughs 22 are found at regular intervals. The parallel rays or convergent rays transmitted through and allowed to exit from the Fresnel lens sheet 1 are diffused widely in the horizontal direction. By this, it becomes possible for an observer to observe an image in a wide visual field in the horizontal direction.
As shown in FIG. 4, plural exit side lenses 23 are arranged on the light exit side of the lenticular lens sheet 2. Among these plural exit side lenses 23, a light absorbing layer 3 made of a light absorbing material such as black ink is disposed. This light absorbing layer 3 covers a region (non-light-condensing region 4) other than light-condensing parts of respective incidence side lenses 21 disposed on the light incidence surface side. Thus, the light absorbing layer 3 is disposed on the light exit side so that the contrast in a light room may be improved. Further, in FIG. 4, boundary portions 31 of the light absorbing layer 3 are disposed on the slanting surfaces 41 of the non-light-condensing region 4.
In a lenticular lens sheet 2 for a projection type image display having plural image light sources, the light-condensing parts on the light incidence surface are sometimes provided with other lenses for the purpose of avoiding color irregularities. In the following, the light incidence surface is sometimes referred to also as “HL surface”, and the light absorbing layer 3 as “black stripes” or “BS”.
In the lenticular lens sheet 2 as such, the relative position between the position of each incidence side lens 21 disposed on the light incidence surface side and the light absorbing layer 3 or between the position of each incidence side lens 21 on the light incidence surface and each exit side lens 23 is important. More specifically, if the light absorbing layer 3 partly covers the light condensing site of an incidence side lens 21 on the light incidence surface side, the transmitted rays are partly blocked and the transmission efficiency may be reduced or the intended exit pattern may not be obtained (cf. Patent Document 1).
Further, if an incidence side lens 21 on the light incidence surface side and the corresponding exit side lens 23 are not in the intended positional relationship, problems will arise; the intended exit pattern may not be obtained or the color irregularity may not be reduced to a satisfactory extent. It is therefore necessary to precisely adjust the positions of the incidence side lenses 21 on the light incidence surface, the exit side lenses 23 and the light absorbing layer 3.
In recent years, in particular, the lenticular lens sheets 2 have been demanded to be higher definition ones and, therefore, it is necessary to make the pitches finer. For example, the pitches in the past were about 0.7 to 1.0 mm and the relative positional misalignment between the lenses and light absorbing layer (hereinafter sometimes referred to also as “optical axis discrepancy”) was within ±10 μm and such accuracy was sufficient. In recent years, however, 0.5-mm or finer pitches have been used and, as for the optical axis discrepancy, accuracies better than ±5 μm have been demanded.
The conventional extrusion molding of a thermo-plastic resin using roll molds or the so-called 2P molding using an ultraviolet-curable resin allows the occurrence of periodic positional misalignments of ±2 to 3 μm in the direction parallel to the axis in view of the performance characteristics of roll-supporting bearings, among others. When the above-mentioned lenticular lens is produced on the conventional production apparatus, the above-mentioned periodic positional misalignment arises between the roll for forming one side of the sheet and the roll for forming the opposite side and the accuracy is about ±5 μm in terms of positional misalignment or optical axis discrepancy between the front and reverse sides.
Patent Document 1: Japanese Kokai Publication H05-150371